Anhydride polymers for use as curing agents in epoxy resin-based underfill material

ABSTRACT

An underfill material is presented that may be used between an electrical component and a substrate. The underfill material may be a cured epoxy resin composition comprising a liquid or semisolid epoxy resin and a polyfunctional anhydride polymer and/or oligomer curing agent. The use of anhydride polymers and/or oligomers decrease the volatilization of the composition, thereby reducing the porosity of the underfill material. By changing substituents of the anhydride polymer and/or oligomer, the underfill material may be designed to modify viscosity, decrease moisture adsorption, volatilization and modulus, improve mechanical properties, and enhance adhesion.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a connection between anelectrical component and a substrate. More particularly, the presentinvention relates to controlling the coefficient of thermal expansionfor an underfill between a chip and a substrate (e.g., in flip chipmounting).

[0002] Microelectronic devices contain millions of electric circuitcomponents, including transistors assembled in integrated circuit chips,resistors, and capacitors. These electronic components areinterconnected to form the circuits, and are eventually connected to andsupported on a substrate. The connections are made between electricalterminations on the electronic component and corresponding electricalterminations on the substrate. One method for making these connectionsincludes a flip-chip mounting technique. Flip chips are made bypositioning the silicon die (“the chip”) with the active side (“theface”) down on the substrate. Bond pads on the face of the chip areconnected by solder bumps or other interconnects to the substrate.During reflow, the solder bumps complete the electrical connections fromthe active circuitry of the die to the substrate.

[0003] During subsequent manufacturing steps, an electronic assembly issubjected to cycles of elevated and lowered temperatures. Because thereis a significant difference in the coefficient of thermal expansion(CTE) for the chip, the interconnect material, and the substrate, thisthermal cycling can stress the components of the assembly and causefailure at the interconnect points, thereby destroying the functionalityof the circuit. To help prevent such failure, the space between the chipand the substrate is underfilled with a dielectric organic material.Once cured, the underfill acts as a buffer between the chip and thesubstrate and functions to distribute the CTE-induced stress over theentire surface, thereby greatly increasing the life of the finishedpackage. Underfill material also protects the interconnects frommoisture and other forms of contamination, and thus, overmolding theback of the chip with epoxy is unnecessary.

[0004] The CTE of the underfill material is critical to the reliabilityof the device because the underfill material compensates for thedifference in CTE between the substrate and the chip. In order to reducesolder joint fatigue and extend solder joint life, the CTE of theunderfill material should be in the range of about 20 to 40 ppm/° C. attemperatures below its glass transition temperature (T_(g)).

[0005] Underfilling may occur after the reflow of the metallic orpolymeric interconnect, or it may occur simultaneously with the reflow.If underfilling occurs after reflow of the interconnect, a predeterminedamount of the underfill material may be dispensed at one or more sidesof the gap between the chip and the substrate. The material will flow bycapillary action into the gap, thereby contacting the solder bumps. Someof the defects that can originate during the flow of fluid underfillinclude delaminations, where the underfill fails to wet and adhere to asurface, and voids, where contamination causes local variations in thespeed of flow and causes bubbles to be trapped. Reducing the viscosityof underfill material, however, enables the material to flow more easilyinto the small gaps between the silicon die and the substrate. Theunderfill material is subsequently cured to reach its optimized finalproperties.

[0006] If underfilling occurs simultaneously with reflow of the solderor polymeric interconnects, the underfill material first is applied toeither the substrate or the chip. Then terminals on the chip andsubstrate are aligned and contacted and the assembly is heated to reflowthe metallic or polymeric interconnect material. During this heatingprocess, curing of the underfill material occurs simultaneously withreflow of the metallic or polymeric interconnect material.

DETAILED DESCRIPTION

[0007] In general an underfill material, when cured, is a compositematerial made up of cross-linked resin. Generally, cross-linking is theattachment of two polymer chains by bridges of an element, a moleculargroup, or a compound, and in general will occur upon heating. Polymerscan be prepared at a variety of cross-link density—from tacky,elastomeric to tough, glassy—by the judicious choice and amount of mono-or polyfunctional compounds, resins, and crosslinking agents. Thegreater proportion of polyfunctional compounds reacted, the greater thecross-link density. If thermoplastic properties are desired, underfillmaterial may be prepared from different compounds to limit thecross-link density. The cross-link density may also be controlled togive a wide range of T_(g) in the cured underfill in order to withstandsubsequent processing and operation temperatures.

[0008] Presently, the anhydrides that are used in underfill material aresmall molecules that have a tendency to volatize during the curingprocess. Such volatilization leads to porosity during underfill cure,which leads to system failure (e.g., delamination, voids, and moisturepenetration). Further, the presently-used anhydrides typically onlyperform one function, i.e. cross-linking.

[0009] An embodiment of the present invention relates to a curableliquid or semisolid underfill material composition comprising resin suchas epoxy resin and silica particles and curing agents such aspolyfunctional anhydride polymers and oligomers. The use of lowmolecular weight anhydride polymers and/or oligomers facilitates thecure process by decreasing the volatilization of the composition, andthereby reducing porosity in the cured underfill materials. Further,using anhydride oligomers and/or polymers in the underfill materialcreates a unique opportunity to design various oligomers and/or polymersthat may react with the epoxy matrix, thereby crosslinking the matrix,and may also provide a structure that may be designed, by substitutingdifferent R groups, to modify, viscosity, decrease moisture absorption,volatilization and modulus, improve mechanical properties, and/orenhance adhesion. The underfill material may also optionally includecatalysts for promoting cross-linking and to control cure time,elastomers for toughening, and/or coupling agents, fillers, fluxingagents, and other additives for flow modification and adhesion. Theunderfill material may also have a higher T_(g), thereby resulting introbust material for 260° C. reflow conditions.

[0010] The underfill material includes resin, which may be present in anamount of from about 25 to about 100 weight percent based on the organiccomponents present. Suitable resins include epoxy resin: such ascycloaliphatic epoxy resins, bisphenol A type epoxy resins, bisphenol-Ftype epoxy resins, novolac epoxy resins, biphenyl type epoxy resins,naphthalene type epoxy resins, dicyclopentadiene-phenol type epoxyresins, and mixtures thereof.

[0011] The curing agents comprise polyfunctional, low molecular weightanhydride polymers and/or oligomers. In accordance with an embodiment ofthis invention the curing agents may include these polymers and/oroligomers in combination with other compounds. The polymers and/oroligomers may be present in the curable underfill material compositionin an amount of from about 5 to about 25 weight percent based on totalweight of the resin and catalysts. Preferred curing agents includeolefin/maleic anhydride copolymers, such as, styrene/maleic anhydride,cyclohexane/maleic anhydride, norbornene/maleic anhydride copolymers. Byway of example, the following chemical schematic shows a low-cost, lowmolecular weight poylmeric cross-linker that may be used in an underfillcomposition:

[0012] where n is 0 to 3, n′ is 5 to 50, and R is selected from thegroup consisting of alkyl, aryl, substituted aryls, esters, ethers,lactones, anhydrides, alcohols, nitriles, epoxy, and mixtures thereof.

[0013] The underfill material composition may include a catalyst,thereby effecting the desired behavior of the formulation. For instance,in addition to controlling the rate of the reaction, catalysts may beused to promote cross-linking and/or to control the curing time of theresin. Suitable catalysts include imidazoles, phosphines, dicyanamide,and substituted dicyamide compounds.

[0014] The anhydride polymers and/or oligomers may contain different Rgroups, which may decrease moisture absorption, volatilization andmodulus, may improve mechanical, properties, and/or may enhanceadhesion. The R group may be nitriles, acids, and/or alcohols. By way ofexample, the following structural schematics show variations ofpolymeric anhydride cross-linkers.

[0015] Preferably, the underfill provides strength to the compositepolymer and to the resin bonds with the chip and substrate. Also, theunderfill material may help to mechanically interlock the chip to thesubstrate, so it may be desirable for the underfill to exhibit goodadhesion to both the die and the substrate. In order to improve theadhesion property of the underfill, coupling agents are widely used.Examples of suitable coupling agents include silanes, titanates,zirconates, aluminates, silicate esters, metal acrylates ormethacrylates, compounds containing a chelating ligand (e.g., phosphine,mercaptan, acetoacetate), and/or mixtures thereof. Certain couplingagents, such as titanate and zirconate, may also catalyze the curingreaction of the epoxy underfill, thereby lowering the onset temperaturesof curing and increasing the viscosity of the underfill material duringstorage at low temperatures. The kinds and amounts of coupling agentsthat may be used are known to those skilled in the art.

[0016] The underfill composition may also include a filler material,which is used to adjust the CTE to more closely mirror that of theinterconnect. Suitable fillers include silica, graphite, aluminumnitride, silicon nitride, silicon carbide, boron nitride, diamond dust,and clays. Typically, the fillers may be present in an amount of about20′ to about 80 weight percent of the total underfill composition.Further, using well-known techniques, the fillers may be treated to makethe filler hydrophobic by silylating agents and/or agents reactive tothe adhesive matrix, such as by using coupling agents.

[0017] The underfill material may also contain compounds that increaseflexibility and toughness in the resultant cured composition. Suchcompounds may be any thermoset or thermoplastic material having a T_(g)of 50° C. or less. Suitable compounds include polyacrylates, polymerizedTHF (tetrahydrofuran), carboxy-terminated butyronitrile rubber, andpolypropylene glycol. When present, these compounds may be in an amountof from about 10 to about 20 weight percent of the epoxy resin.

[0018] Siloxanes may also be added to the underfill materialcompositions to provide elastomeric properties. Suitable elastomersinclude methacryloxypropyl-terminated polydimethyl siloxanes, andarinopropyl-terminated polydimethylsiloxanes. Other additives, such asfluxing agents, may also be added as needed. The kinds and amounts ofelastomers and fluxing agents that may be used are known to thoseskilled in the art.

[0019] In general, the underfill material may be cured within atemperature range of from about 130° C. to about 170° C. and withinabout 5 minutes to about 4 hours. The time and temperature curingprofile for each underfill material, however, may vary.

[0020] The underfill material may be prepared, for example, bysimultaneously or separately agitating, dissolving, mixing anddispersing the epoxy resin, and curing agent, and optionally catalysts,elastomers, coupling agents, fillers, fluxing agents and/or flow andadhesion agents. The device for mixing, agitating, and dispersing is notcritical although, an attritor, three-roll mill, ball mill or planetarymixer each equipped with agitating and heating means may generally beused. A suitable combination of these devices may also be useful.

[0021] The curable underfill material composition containing lowmolecular weight anhydride polymers and/or oligomers may be appliedusing a standard capillary flow process or assisted flow process. Theunderfill material may be used for new lead free under bump metallurgy(i.e., SnAgCu, SnAg, SnCu) and may be applied with other newer processessuch as no-flow underfills, which, if properly formulated, maysignificantly decrease manufacturing cost by eliminating the fluxingprocess, the underfill flow process, and the underfill cure process.According to an embodiment of the present invention, such material maybe used with lead-free solder materials or with an instant chip joiningprocess.

EXAMPLES

[0022] Examples of the invention are given below by way of illustrationand not by way of limitation.

Example 1

[0023] Styrene/maleic anhydride copolymer (2 wt %) of a molecular weightof about 1600 g/mole and the catalyst imidazole (1 wt %) were added tobis F resin (4 grams). Scheme 6 shows the structure of the composition.The resin was heated to about 70° C. and mixed thoroughly until all ofthe polymer was observed to dissolve in the epoxy resin, thereby forminga homogeneous solution. The solution was cooled to room temperature andcured at about 165° C. for about 2 hours. A cross-linked hardtransparent polymer was obtained.

Example 2

[0024] Styrene/maleic anhydride copolymer (5 Wt %) of a molecular weightof about 1600 g/mole and the catalyst imidazole (1 wt %) were added tobis F resin (4 grams). Using the same process as described in Example 1,a cross-linked hard transparent polymer was obtained.

Example 3

[0025] Styrene/maleic anhydride copolymer (7.5 wt %) of a molecularweight of about 1600 g/mole and the catalyst imidazole (1 wt %) wereadded to his F resin (4 grams). Using the same process as described inExample 1, a cross-linked hard transparent polymer was obtained.

Example 4

[0026] Styrene/maleic anhydride copolymer (10 wt %) of a molecularweight of about 1600 g/mole and the catalyst imidazole (1 wt %) wereadded to bis F resin (4 grams). Using the same process as described inExample 1, a cross-linked hard transparent polymer was obtained. By DMA(Dynamic Mechanical Analysis), the of the polymer was found to be 55° C.Using TMA (Thermal Mechanical Analysis) measurements ₇ the coefficientof thermal expansion of the polymeric system was found to be 63 ppm/° C.

Example 5

[0027] Styrene/maleic anhydride copolymer (25 wt %) of a molecularweight of about 1600 g/mole and the catalyst imidazole (1 wt %) wereadded to bis F resin (4 grams). Using the same process as described inExample 1, a cross-linked hard transparent polymer was obtained. By DMA,the T_(g) of the polymer was found to be 75° C. Using TMA measurements,the coefficient of thermal expansion of the polymeric system was foundto be 57 ppm/° C.

Example 6

[0028] To determine the percent weight during cure, the followingexperiment was conducted.

[0029] Styrene/maleic anhydride copolymer (10 wt %) of a molecularweight of about 1600 g/mole and the catalyst imidazole (1 wt %) wereadded to bis F resin (4.0 grams). The resin was mixed thoroughly to forma homogeneous solution. A small amount of the resin was heated fromabout 25° C. to about 165° C. and held for 2 hours usingthermogravimetric analysis. Weight loss during cure was measured to be 1wt %.

[0030] A similar experiment was conducted with a monomeric anhydridesample. Cyclohexyl anhydride copolymer (1.18 grams) and the catalystimidazole (1 wt %) were added to bis F resin (2.0 grams). Weight lossduring cure was measured to be 3.6 wt %.

Example 7

[0031] Cyclic olefin (10 wt %) containing various, functional groups(e.g., epoxies, esters, ethers, alcohols, anhydrides, nitriles)/maleiccopolymer of a molecular weight <5000 g/mole and the catalyst imidazole(1%/wt) are added to epoxy resin (4 grams) such as bis F, napthelene,and the like. The resin is heated to about 70° C. and mixed thoroughlyuntil all of the polymer is observed to form a homogeneous solution. Thesolution is cooled to room temperature and cured at about 165° C. forabout 2 hours. A cross-linked hard hazy polymer is obtained.

Example 8 Use of Polymeric Anhydride Formulation as Underfill Material

[0032] The product in example 4 was evaluated as an underfill on a flipchip package. The product was dispensed on one side of a die of aflip-chip package that had been pre-baked. The substrate was held atabout 70° C. for about 1 minute for the underfill to flow through thegap between the die and the substrate. After 1 minute, the package, wasremoved and wet CSAM (C-mode Scanning Acoustic Microscopy) was conductedto confirm full coverage of the underfill and also to confirm absence ofany voids. The unit was later cured at about 165° C. for about 2 hours,thereby curing the underfill material. The fillet′(underfill around thedie) was confirmed to be completely cured. CSAM of the package post curewas also observed to be clean with no voiding or delamination as shownbelow.

[0033] Although embodiments are specifically illustrated and describedherein; it is to be appreciated that modifications and variations of thepresent invention are covered by the above teachings and are within thepurview of the appended claims, without departing from the spirit andintended scope of the invention.

1. An underfill composition comprising: a resin; and a curing agentselected from the group consisting of anhydride polymers, anhydrideoligomers, anhydride copolymers and mixtures thereof.
 2. The compositionaccording to claim 1 wherein the curing agent is an olefin/maleicanhydride.
 3. The composition according to claim 2 wherein theolefin/maleic anhydride is selected from the group consisting ofstyrene/maleic anhydride, cyclohexane/maleic anhydride, andnorbornene/maleic anhydride.
 4. The composition according to claim 1further comprising at least one catalyst, elastomer, coupling agent,filler, fluxing agent, flow agent, adhesion agent, and mixtures thereof.5. The composition according to claim 4 wherein the catalyst is selectedfrom the group consisting of imidazoles, phosphines, dicyanamide, andsubstituted dicyamide compounds.
 6. The composition according to claim 4wherein the curing agent is in an amount of from about 5 to about 25weight percent based on total weight of the resin and the catalyst. 7.An underfill material that is a cured epoxy resin compositioncomprising: one of a liquid and semisolid epoxy resin; and a curingagent selected from the group consisting of anhydride polymers,anhydride oligomers, anhydride copolymers, and mixtures thereof.
 8. Thecomposition according to claim 7 wherein the curing agent is anolefin/maleic anhydride.
 9. The composition according to claim 8 whereinthe olefin/maleic anhydride is selected from the group consisting ofstyrene/maleic anhydride, cyclohexane/maleic anhydride, andnorbornene/maleic anhydride.
 10. The composition according to claim 7further comprising at least one catalyst, elastomer, coupling agent,filler, fluxing agent, flow agent, adhesion agent, and mixtures thereof.11. The composition according to claim 10 wherein the catalyst isselected from the group consisting of imidazoles, phosphines,dicyanamide, and substituted dicyamide compounds.
 12. The compositionaccording to claim 10 wherein the curing agent is in an amount of fromabout 5 to about 25 weight percent based on total weight of the resinand the catalyst.
 13. A device comprising: a substrate; an electricalcomponent; and an underfill composition between the electrical componentand the substrate, the underfill composition including a resin; and acuring agent selected from the group consisting of anhydride polymers,anhydride oligomer, anhydride copolymers and mixtures thereof.
 14. Thedevice according to claim 13 wherein the curing agent is anolefin/maleic anhydride.
 15. The device according to claim 14 whereinthe olefin/maleic anhydride is selected from the group consisting ofstyrene/maleic anhydride, cyclohexane/maleic anhydride, andnorbornene/maleic anhydride.
 16. The device according to claim 13further comprising at least one catalyst, elastomer, coupling agent;filler, fluxing agent, flow agent, adhesion agent, and mixtures thereof.17. The device according to claim 16 wherein the catalyst is selectedfrom the group consisting of imidazoles, phosphines, dicyanamide, andsubstituted dicyamide compounds.
 18. The device according to claim 16wherein the curing agent is in an amount of from about 5 to about 25weight percent based on total weight of the resin and the catalyst. 19.A method of fabricating a device, comprising: fabricating an integratedcircuit chip, the integrated circuit chip including a plurality ofelectrical bond pads; fabricating a substrate, positioning theintegrated circuit chip relative to the substrate; providing electricalconnection between the integrated circuit chip and the substrate duringa reflow operation; providing an underfill composition between theintegrated circuit chip and the substrate, the underfill compositionincluding a resin; and a curing agent selected from the group consistingof anhydride polymers, anhydride oligomers, anhydride copolymers, and amixture thereof.
 20. The method according to claim 19 wherein theunderfill composition is provided simultaneously during reflow.
 21. Themethod according to claim 19 wherein the underfill composition isprovided after reflow.
 22. The method according to claim 19 wherein theunderfill composition is cured.
 23. The method according to claim 22wherein the curing occurs within a temperature range of from about 130°C. to about 170° C.
 24. A method according to claim 22 wherein thecuring occurs within about 5 minutes to about 4 hours.